More American women are putting off becoming moms until after their best child-bearing years, a shift that reflects both the expanded role of women in the workforce and the nation’s shrinking fertility rate.

In 1970, just 1% of first children were born to women over the age of 35. In 2012, 15% were, according to an analysis by Gretchen Livingston of the Pew Research Center of data from the Centers for Disease Control & Prevention.

The findings, along with new data from the CDC released Friday, show the trend of delayed childbearing among women in their late thirties and early forties continuing to pick up, with big implications for American society and the economy.

Delayed childbearing has allowed more women to pursue educations and careers, and achieve some measure of financial independence–things that can buffer women from major shocks like an economic downturn or a failed marriage, demographers say.

“This is good news,” says Philip Cohen, a sociologist at the University of Maryland. “They are more protected against the shock of earnings loss, and they’re probably more independent and better-established in their relationships.”

At the same time, women who have kids much later in life also face increased health risks, both to themselves and their infants–especially mothers over 40, the CDC notes. The average age when a U.S. woman has her first child was 25.8 years old in 2012, up from 21.4 in 1970.

If women have their first kid in their late thirties or early forties, they’re also more likely to have smaller families, pushing down the nation’s fertility, which hit a new low in 2012. Lower fertility, in turn, can reduce the economy’s long-term growth potential by cutting the size of the productive workforce and leaving fewer people to support the elderly.

In their report Friday, the CDC highlighted just how resilient the trend of delayed childbearing has been over time.

The number of first children per 1,000 women between the ages of 35 and 39 rose nearly 25% between 2000 and 2012. In other words, for every 1,000 women of this age, there were 11 “first births,” up from 8.9 in 2000. Among women 40 to 44, there was a bigger percentage jump, of 35%, to 2.3 births, from 1.7 births.

All told, there were roughly nine times as many first children born to women 35 and up in 2012 as there were four decades earlier, the CDC said.

Kim Jalet of Somerville, Massachusetts, near Boston, is one of those moms.

Last July, Ms. Jalet, 40, and her 43-year-old husband had their first child, a daughter, after being married for nearly two decades.

For years, the couple was busy with their jobs–Ms. Jalet was a teacher in inner-city Boston, and also enjoyed the freedom of being childless. “We decided we really had a nice life and liked the way it was,” Ms. Jalet said.

Two years ago, though, they figured if they were going to have a child, they better start trying. If it didn’t work out, they were fine too, Ms. Jalet says.

Now, having worked as a teacher for 18 or so years, Ms. Jalet says she doesn’t feel like she’s missing out by devoting her days to her daughter. “I haven’t been out for a year, but that’s okay for me, she says. “It’s a matter of when you’re ready.”

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